The present invention relates to a pretreatment method for the production of a fire retardant foam material. More particularly, the present invention relates to a pretreatment method in which solid particles of urea are mixed with one or more of the foam producing reactants prior to combining or at the time of combining such reactants to produce a fire retardant flexible polyurethane foam.
In the production of flexible polyurethane foam materials, a polyol such as polypropylene glycol is typically treated with a polyisocyanate in the presence of water and a catalyst. As the polymer forms, water reacts with the isocyanate groups to produce carbon dioxide which causes foaming. In some cases a blowing agent may be employed to produce the foaming effect.
In U.S. patent application Ser. No. 412,441 filed Aug. 27, 1982 there is described a method for production of fire retardant foam such as flexible or semi-flexible polyurethane foam in which various materials utilized to provide fire retardancy are incorporated after the foam mass is produced.
By the present invention, there is provided an improved method for the production of foam materials having a high degree of fire retardancy, utilizing a pretreatment method in which the materials utilized to promote fire retardancy are incorporated with the reactant materials prior to or during production of the foam mass, thus eliminating the necessity for a second operation. The present invention is an improvement upon current formulations and methods utilized in the foam industry, particularly the urethane industry, in which pretreatment methods are employed which include the use of halogenated phosphorus compounds, antimony oxide and alumina trihydrate. Such prior art formulations have been found to decrease the physical quality of the foam, produce questionable levels of fire retardancy and also to be very expensive to produce. The present invention, on the other hand, results in a highly fire retardant foam which can be produced at low cost and without impairing the physical qualities of the foam.
In one embodiment, the urea and the foam producing materials are mixed in a slurry apparatus whereby the urea is pre-mixed with the polyol component of the foam prior to the addition of the isocyanate. In an alternative embodiment, the urea is incorporated into the reacting mass of polyol and isocyanate in solid form at the mixing point, such as by injection into a mixing head with an endless screw conveyor. This latter method is particularly useful in a case where the polyols which are employed are primarily capped with ethylene oxide which tends to dissolve the urea to a point where it can not be processed through typical pumping and metering equipment such as are employed in the slurry method.
In the case where polyols which are primarily capped with ethylene oxide are employed, the urea component should be incorporated into the reacting mass in solid form at the mixing point. When an endless screw conveyor is used for this purpose, the urea enters the mixing chamber from the conveyor and is evenly dispersed in the foam mass. The screw conveyor may operate at speeds such as from 2500 to 9000 rpm. In this embodiment, the urea enters the screw chamber from a pressurized container which forces the urea into the chamber. The dispensing mechanism is synchronized by means of a solenoid valve that activates the screw at the same time that the reacting liquids are introduced into the mixing chamber.
In adding the urea to the foam producing materials, according to the invention, the urea component may be effectively employed in amounts of from about 30 to 45% by weight of the total foam mass composition, i.e., foam and urea. The minimum desirable level is about 30% by weight of urea and optimum fire retardancy was achieved by the use of about 40 to 45% by weight urea in the composition. Thus, for example, a foam mass prepared with 40% urea exhibited a Flame Spread Index level of less than 25 when treated in accordance with ASTM-E-162 Radiant Panel Test. The same foam mass with no urea yielded a Flame Spread Index of more than 1000. It has been found that the presence of urea in the foam mass does not appreciably affect either the tensile strength, tear value or compression set values of the foam.